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© The Rockefeller University Press,
0021-9525/2000//691 $5.00
The Journal of Cell Biology, Volume 148, Number 4,
, 2000 691-702
Original Article |
Interaction among Gsk-3, Gbp, Axin, and APC in Xenopus Axis Specification
kimelman{at}u.washington.edu
Glycogen synthase kinase 3 (GSK-3) is a constitutively active kinase that negatively regulates its substrates, one of which is β-catenin, a downstream effector of the Wnt signaling pathway that is required for dorsal–ventral axis specification in the Xenopus embryo. GSK-3 activity is regulated through the opposing activities of multiple proteins. Axin, GSK-3, and β-catenin form a complex that promotes the GSK-3–mediated phosphorylation and subsequent degradation of β-catenin. Adenomatous polyposis coli (APC) joins the complex and downregulates β-catenin in mammalian cells, but its role in Xenopus is less clear. In contrast, GBP, which is required for axis formation in Xenopus, binds and inhibits GSK-3. We show here that GSK-3 binding protein (GBP) inhibits GSK-3, in part, by preventing Axin from binding GSK-3. Similarly, we present evidence that a dominant-negative GSK-3 mutant, which causes the same effects as GBP, keeps endogenous GSK-3 from binding to Axin. We show that GBP also functions by preventing the GSK-3–mediated phosphorylation of a protein substrate without eliminating its catalytic activity. Finally, we show that the previously demonstrated axis-inducing property of overexpressed APC is attributable to its ability to stabilize cytoplasmic β-catenin levels, demonstrating that APC is impinging upon the canonical Wnt pathway in this model system. These results contribute to our growing understanding of how GSK-3 regulation in the early embryo leads to regional differences in β-catenin levels and establishment of the dorsal axis.
Key Words: Wnt pathway • dorsal/ventral • β-catenin
© 2000 The Rockefeller University Press
G.H. Farr III and D.M. Ferkey contributed equally to this work.C. Yost's present address is Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, WA 98109-1024. S.B. Pierce's present address is Department of Molecular Biology, Wellman 8, Massachusetts General Hospital, Boston, MA 02114.
Abbreviations used in this paper: APC, adenomatous polyposis coli; dnXgsk-3, dominant-negative Xgsk-3; GSK, glycogen synthase kinase 3; GST, glutathione-S-transferase; MBP, maltose binding protein; Xgsk-3, Xenopus GSK-3.
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